Flat-panel loudspeakers as such have been known for a long time, for example from DE 484 872. In the case of a flat-panel loudspeaker, a moving coil working on the electrodynamic principle is used and it is placed directly on a surface—in principle, initially of any desired size and thickness and consisting of a chosen material—and mechanically fixed thereon. If the moving coil is electrically excited by a source of sound, its vibrations are transmitted to the surface acting as a diaphragm and this surface is thus itself used as a sound-radiating surface. For an electroacoustic transducer of this generic type, there would actually be a large number of possible uses. However, if to date it has not gained widespread acceptance, apart from a few exceptions, this is due to its electroacoustic properties, in particular its transfer function.
The mechanical properties of the sound-radiating surface are important. This surface can only transmit tones or sounds when it mechanically vibrates. Apart from the mounting, i.e. the mechanical support and the place where the moving coil is fixed on it, a plate-shaped surface that is excited preferably to vibrate flexurally is in its vibration response actually already a relatively complex structure. While in the case of a commercially available loudspeaker working on the electrodynamic principle it is still largely within one's control to optimize the sound-radiating diaphragm with regard to its acoustic properties, albeit with compromises, this is not readily possible in the case of a flat-panel loudspeaker. This problem may be illustrated by means of an example: If a glass surface of a show window, on which surface a moving coil is placed, is used as a flat-panel loudspeaker, the material, shape and dimensions of the sound-radiating surface, as well as its mounting, are substantially fixed. The frequency response of the flat-panel loudspeaker in this example is thus substantially predetermined. Typically, the natural resonances of the surface utilized for the sound radiation with this material and given the dimensions of the show window cause a frequency response which is to be described—simplified—by an excessive reproduction in the range of low tones and furthermore by a rattling tendency, which is due to the influence of higher-order natural resonances still lying in the audible range. Corresponding characteristic nonlinearities also occur with other materials, such as wood or plastic materials.
As is known, for example, from U.S. Pat. No. 3,728,497, U.S. Pat. No. 3,636,281 or U.S. Pat. No. 3,449,531, efforts have been made to overcome the known disadvantages of the flat-panel loudspeaker by means of constructional measures. It has been possible to achieve certain improvements in this manner, but a fundamental solution which would have opened up a wide range of applications for the flat-panel loudspeaker has not yet been achieved by the attempts made to date.
From psychoacoustics, it is known that for intellectual work there is an increased degree of difficulty when the noise level in the working environment is increased. A further aspect is working in a room with several people sitting close to another when they are having conversations at the same time, as occurs for example in a call centre. Here, it is very disturbing when the neighbor's voice can be clearly understood. Unintentionally overhearing the contents of other people's conversations greatly affects one's own powers of concentration when doing intellectual work. Simply by not being able to clearly understand these voices will increase the quality of work owing to the improved intellectual concentration.